1. Field of the Invention
The present invention relates to an apparatus for depositing a silicon-based thin film and a method for depositing a silicon-based thin film.
2. Description of the Related Art
Silicon-based thin films are expected to be used for promising solar cells in view of constraints of silicon resources, etc. With respect to amorphous silicon thin films which have been used to date, solar cells with a conversion efficiency of about 8% have been put into practical use. Recently, tandem solar cells having structures in which amorphous silicon and thin film crystalline silicon are stacked on one another have been developed. In these tandem cells, by using solar light in a wide wavelength range, photovoltaic power generation with a conversion efficiency of about 15% is becoming possible. As a method for forming such thin-film structures, decomposition and deposition of a silane-based gas using a PECVD (Plasma Enhanced Chemical Vapor Deposition) apparatus are generally performed. Amorphous silicon are required to have a thickness of about several hundred nanometers, and thin film crystalline silicon are required to have a thickness of about several thousand nanometers. Consequently, in order to achieve both high deposition ratio and good film thickness uniformity, various proposals have been made regarding PECVD apparatuses.
For example, in low pressure PECVD at a pressure of 1 kPa or less, in order to achieve high deposition ratio and improve film thickness distribution on a large-area substrate, methods in which a radio frequency voltage is intermittently applied to perform discharge (short pulse VHF PECVD method) have been employed in recent years. Japanese Patent No. 3420960 discloses an example of short pulse VHF PECVD method, in which two pairs of cathode and anode electrodes are provided in a chamber, a base material on which a film is formed is placed on each cathode electrode set at a ground potential, a mixed gas of silane and hydrogen is introduced into the chamber, and short pulse VHF waves are applied to the individual anode electrodes so that their ON periods do not coincide with each other, thereby depositing silicon-based thin films on the base materials. The discharge is carried out under the following conditions: frequency of VHF voltage: 27.12 MHz, ON period of modulated pulse: 10 μsec, duty ratio: 20%, and shift of ON periods: 25 μsec.